When you have taken endurance damage, go to the inventory screen, and select "Armor". Re-equip your armor (highlight armor and press X) to completely refill your endurance meter. Repeat this as many times as desired. -From: Ezra Rainstein

Easy ammunition in "Intercept Nasri The Arms Dealer" mission

Successfully complete the "Defeat Nasri's Guards" objective, and remain in that room. There are three sets of stairs, with a door behind two of them and another with a table across the top. In front of that table is an random ammunition collectable. This collectable will respawn after approximately thirty seconds.

Easy ammunition in "Investigate Marburg's Villa" mission

At the end of the mission, you can find two ammo collectibles in the garden on the north and south sides. They will respawn after you move back to the doors.

Easy Boss kills

Get the pistol up to the advanced chain shot level. When a Boss sticks their head out, do a five shot chain shot to kill it instantly.

Customizing appearance

You can customize your character's appearance again by going to the bathroom.